1. Field of the Invention
This invention relates generally to a system and method for determining whether contacts in a high voltage (HV) contactor used in a battery circuit have been welded or stuck closed and, more particularly, to a battery circuit for an electric vehicle (EV) or a hybrid electric vehicle (HEV) that includes circuit components for determining if contacts in an HV contactor used in the circuit have been welded or stuck closed.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV) that combine a battery and a main power source, such as an internal combustion engine, fuel cell systems, etc., and electric only vehicles, such as the battery electric vehicles (BEV). All of these types of electric vehicles employ a high voltage battery that includes a number of battery cells. These batteries can be different battery types, such as lithium ion, nickel metal hydride, lead acid, etc. A typical high voltage battery system for an electric vehicle may include a large number of battery cells or modules including several battery cells to meet the vehicle power and energy requirements. The battery system can include individual battery modules where each battery module may include a certain number of battery cells, such as twelve cells. The individual battery cells may be electrically coupled in series, or a series of cells may be electrically coupled in parallel, where a number of cells in the module are connected in series and each module is electrically coupled to the other modules in parallel. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application.
The high voltage battery in an electric vehicle is selectively coupled to the vehicle's high voltage bus by HV battery contactors. When the vehicle is shut off, the contactors are opened and the battery is disconnected from the high voltage bus. When the vehicle is switched on, the contactors are closed and the battery voltage is coupled to the high voltage bus.
Battery circuits of the type discussed above typically employ several HV contactors, including the battery disconnect contactors mentioned above, that are capable of operating with high current for the particular application. The contacts in the HV contactors are typically held normally open by a spring where a coil in the HV contactor is energized to move the contacts against the bias of the spring to close the HV contactor. Because of the high current that is passing through the HV contactor, it is possible that the contacts that make the connection in the HV contactor when the HV contactor is closed will arc possibly causing the contacts to essentially weld together, where the contacts remain closed and an electrical connection through the HV contactor is still made after the coil is de-energized and the HV contactor should otherwise be opened by the spring. Also, this welding could cause a partial closing of the HV contactor where a high resistance across the contactor occurs.
The designers of battery circuits for these high voltage applications consider this problem and provide suitable design features so that an HV contactor that is welded or stuck closed is not likely to be a safety concern. However, such stuck closed HV contactors could still provide high voltage to unwanted parts of the circuit at unwanted times. Algorithms and processes may be employed to detect a welded or stuck HV contactor. These processes typically use the sequencing of the switches to identify a welded or stuck closed HV contactor, where if a high voltage is detected at a location where it shouldn't be, a welded or stuck closed contact may be suspected. If such a welded or stuck closed HV contactor is detected, then the algorithms may provide suitable warnings to the vehicle driver that service is required. Further, because the vehicle battery industry is moving towards lithium-ion batteries, which are desirable because they typically provide a higher performance because of a higher current density than other battery types, the welding problem of the HV contactor contacts increases with these types of batteries.
The above described technique of detecting welded or stuck closed contacts in an HV contactor has various disadvantages including the time factor involved in going through the sequence of opening and closing the HV contactors, measuring the voltage, and determining that an HV contactor is welded or stuck closed. Further, when using this process of sequencing to determine welded or stuck closed contacts, it is typically not possible to determine if the first HV contactor that is closed during vehicle or system start-up is welded or stuck closed.